JP2938445B1 - Quantum well optical modulator, optical communication module and optical communication system using the same - Google Patents

Abstract

Abstract: In a conventional quantum well composed of only III-V group, the extinction ratio deteriorates when the well width is widened because the oscillator strength of excitons is not sufficient. SOLUTION: A material having a large dielectric constant (III-
A quantum well structure is formed by using a material having a low dielectric constant (eg, a group II-VI compound) such as ZeSe for the barrier layer 12, and a ratio of the dielectric constant between the well layer 11 and the barrier layer 12. To be greater than 1.5 to enhance the Coulomb interaction between electrons and holes in the quantum well and increase exciton binding energy and oscillator strength. Then, the quantum well width when the binding energy of the excitons is kept constant is made larger than that of the conventional material system to lower the operating voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantum well optical modulator of a refractive index control type and an electro-absorption type, and more particularly to a quantum well optical modulator suitable for performing high-speed response with low power consumption and the same. The present invention relates to an optical communication module and an optical communication system used.

[0002]

2. Description of the Related Art In recent years, optical transmission technology has made remarkable progress, and an optical transmission system of 10 Gbps (10 gigabit / second) has been put to practical use until now, and it is expected that the capacity will be further increased in the future. You. In this 10 Gbps optical transmission system, the direct modulation technology that changes the drive current of the semiconductor laser and directly modulates the intensity of the output light is replaced by an external modulation technology that enables high-speed operation with low chirping (output from the light-emitting element). Light is intensity-modulated by an external modulator (external modulator).

In this external modulation technique, development of a higher performance external optical modulator has become important in order to further increase the capacity of an optical transmission system. As a material of the optical modulator used for the external modulation, there are a material using a dielectric such as LiNbO _{3 and} a material using a semiconductor such as InP or GaAs. An optical modulator using this semiconductor can be integrated with other optical elements and electronic circuits such as a semiconductor laser, and expectations for a semiconductor optical modulator that can be easily miniaturized and reduced in voltage are increasing. .

[0004] As typical semiconductor optical modulators, there are an electroabsorption optical modulator and a Mach-Zehnder optical modulator. In an electro-absorption optical modulator, when an electric field is applied, the fundamental absorption edge shifts to a longer wavelength side, such as the Franz-Keldysh effect of a bulk semiconductor or the quantum confined Stark effect (QCSE) found in a quantum well structure. Light modulation is performed by controlling the degree of light absorption using the phenomenon. Further, the Mach-Zehnder modulator modulates light by controlling the refractive index with an applied electric field, utilizing the Pockels effect of a bulk semiconductor or a change in the refractive index caused by QCSE.

[0005] An optical modulator employing a QCSE employing a quantum well structure has much higher efficiency than a bulk semiconductor modulator in both the electroabsorption type and the refractive index control type. It has been actively studied as a modulator for optical communication. However, even with these optical modulators using QCSE, the operating voltage under ultra-high-speed operation of 40 Gbps or more is several volts, and from the viewpoint of the reliability and cost of the optical communication system, this operating voltage is greatly increased. Reduction is desired.

Since the effect of QCSE is approximately proportional to the fourth power of the well width, the efficiency of the modulation operation with respect to the electric field can be increased by increasing the quantum well width, that is, the operating voltage can be reduced. However, on the other hand, when the quantum well width is wide, the overlap between the wave functions of electrons and holes is reduced, and the oscillator strength of excitons (electron-hole pairs) is reduced, and as a result, the quenching intensity is also reduced. There's a problem.

As a conventional technique, regarding an electro-absorption type optical modulator, a modulator using an InGaAsP / InGaAs multiple quantum well as an absorption layer has been reported by Miyazaki et al. 4, pages 4-218 ". In addition, a modulator using an InGaAs / InAlAs multiple quantum well as an absorption layer was proposed by Well et al. In "The 1994 IEICE Spring Conference Lectures, Volume 4, pages 4-221".
Has been reported to. Further, as an example of the Mach-Zehnder optical modulator, a modulator using a waveguide layer of InGaAs / InAlAs multiple quantum wells is described by Sano et al. In "1993 IEICE Spring Conference Lecture Volume 4, 4-186. Page ".

[0008]

The problem to be solved is that the prior art employs a quantum well structure and a QCS.
In an electroabsorption type and refractive index control type optical modulator using E (quantum well optical modulator), when the quantum well width is increased, the operating voltage can be reduced, but the wave function of electrons and holes overlaps. In this case, the exciter vibrator strength decreases, and the extinction strength decreases. An object of the present invention is to solve these problems of the prior art, to increase the basic absorption edge shift by widening the quantum well width without lowering the extinction intensity, and to realize a quantum well optical modulator capable of operating at a low voltage and a quantum well optical modulator. An object of the present invention is to provide an optical communication module and an optical communication system used.

[0009]

In order to achieve the above object, a quantum well optical modulator according to the present invention uses a material having a high dielectric constant (a group III-V compound, for example, InGaAsP) for a well layer and a barrier layer. For the layer, use a material with a small dielectric constant such as ZeSe (II
-VI compound) to increase the Coulomb interaction between electrons and holes in the quantum well by increasing the dielectric constant ratio between the well layer and the barrier layer to greater than 1.5, Increase the exciton binding energy and oscillator strength. As a result, the quantum well width when the exciton binding energy is kept constant can be made larger and the operating voltage can be made lower than that of the conventional material system, so that the QCSE has a great effect.

When InP is used for the substrate, ZnCdSeTe or ZnMgSSe is used for the barrier layer. When GaAs is used for the substrate, ZnCdSSe or Zn is used for the barrier layer.
MgSSe may be used. Further, by using GaInNAs having a large dielectric constant for the well layer, the effect of enhancing the oscillator strength of the excitons is further increased, and an optical modulator with higher sensitivity can be obtained.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a perspective view showing one embodiment of the configuration according to the present invention of the quantum well optical modulator of the present invention.
FIG. 1 shows an example of an electro-absorption type quantum well optical modulator. In FIG. 1A, reference numeral 1 denotes an n-InP (100) substrate;
2 is an n-InP cladding layer, 3 is InGaAsP / ZnCdSS
e Multiple quantum well light absorption layer (hereinafter referred to as "MQW light absorption layer"), 4 is a p-InP cladding layer, 5 is p ⁺ -InGaAs.
The layer, 6 indicates an n-electrode, and 7 indicates a p-electrode. Also, 8
Indicates an SiO ₂ layer, and 9 indicates polyimide. Reference numeral 10 denotes an antireflection film applied to both end faces.

The MQW light absorbing layer 3 is composed of an InGaAsP well layer 11 and a ZnCdSSe barrier layer 12, as shown in FIG. In this example, the well width of the InGaAsP well layer 11 is 14 nm, and the barrier width of the ZnCdSSe barrier layer 12 is 5 nm. The characteristic of this example is that InGa
That the AsP well layer 11 has a well width of 14 nm.
The method of manufacturing the quantum well optical modulator itself and the operation of each unit are the same as those of the prior art, and the details will not be described here.

That is, as shown in FIG. 2, the MQW light absorption layer in the conventional quantum well optical modulator is composed of InGaAs / InAlAs, and the InGaAs / InAlAs.
The InGaAs well layer of the As multiple quantum well light absorption layer 13 is 9n.
The m and InAlAs barrier layers have a width of 5 nm. As described above, in the case of the conventional MQW light absorption layer composed of InGaAs / InAlAs, the InGaAs well layer has a width of 9 nm and the InAlAs barrier layer has a width of 5 nm. In this example, the well width of the InGaAs well layer 11 is 14 nm. And, it is wide. Thereby, the operating voltage of the modulation operation can be reduced.

When the well width of the InGaAs well layer 11 is changed from 9 nm to 14 nm in the conventional art, that is, while the InAlAs barrier layer remains, the overlap between the electron and hole wave functions in the well layer becomes small, and the vibration of the exciton is reduced. In this example, this problem is avoided by using ZnCdSSe, which is a II-VI compound, for the barrier layer of the quantum well of the light absorption layer. Hereinafter, the use of the ZnCdSSe barrier layer 12 will be described.

[0015] According to Keldysh, "JETP letters vol. 29 6
P. 58 (1979) ”, Coulomb interaction is enhanced in a thin film of a large dielectric constant sandwiched between materials of a small dielectric constant, so that a material with a small dielectric constant is used for the barrier layer and a dielectric is used for the well layer. It is described that in a quantum well using a material having a high rate, the Coulomb interaction between electrons and holes is enhanced, and the binding energy of an exciton and the oscillator strength are increased.

In a conventional material-based quantum well in which a group III-V compound is used for both the well layer and the barrier layer, the permittivity of the barrier layer and the well layer is about "11" to "14". But the effect of enhancement is hardly seen. On the other hand, II-VI group compounds generally have a small dielectric constant, for example, ZeSe is "7.6". Therefore, II-
When the group VI compound is used, the ratio of the dielectric constant between the well layer and the barrier layer increases, and the binding energy of excitons is increased. Conversely, when the exciton binding energy is kept constant, the quantum well width can be made larger than that of the conventional material system.
E also has a great effect.

In this example, in the quantum well of the optical modulator,
By using ZnCdSSe, which is a II-VI group compound semiconductor, for the barrier layer, the ratio of the dielectric constant εw of the well layer to the dielectric constant εb of the barrier layer is larger than 1.5, that is, (εw / εb)>
It is set to 1.5.

FIG. 3 is an explanatory diagram showing the relationship between the dielectric constant of the well layer and the dielectric constant of the barrier layer relating to the exciton binding energy. In this figure, the permittivity εw of the well layer is fixed to “10” and only the permittivity εb of the barrier layer is changed (“1, 2, 5, 1
0 ") shows the well width dependence of the exciton binding energy. Decreasing the dielectric constant εb of the barrier layer (1
0 → 5 → 2 → 1), it is shown that the binding energy of excitons increases by increasing the ratio between the dielectric constant εw of the well layer and the dielectric constant εb of the barrier layer.

FIG. 4 is an explanatory diagram showing a comparative example of the binding energy between the material used for the quantum well optical modulator in FIG. 1 and the material used in the prior art. In this example, the InGaAsP / ZnCdSSe-based quantum well used for the quantum well optical modulator shown in FIG. 1 and the InGaAs used for the conventional quantum well optical modulator were used.
7 specifically shows the relationship between the well width and the exciton binding energy of a GaAs / InAlAs quantum well.

FIG. 1 shows a comparison in the vicinity of the well width of 9 nm.
InGaAsP / ZnCd used for quantum well optical modulators in Japan
The binding energy of the excitons in the SSe quantum well is the same as that of InGaAs / InAlAs used in the conventional quantum well optical modulator.
The binding energy of the exciton in the system quantum well is increased by about 20%. Conversely, InGaAs / InAl with a well width of 9 nm
InGa of the same exciton binding energy as the As quantum well
The well width of the AsP / ZnCdSSe quantum well is 14 nm. The effect of the QCSE is approximately the fourth power of the well width when the exciton binding energy is the same, so that when the material system of this example is used, “(14/9) ⁴ = 5.9” It can be seen that about six times the sensitivity can be obtained.

Next, an example of an optical modulation communication module using such a quantum well optical modulator will be described. FIG.
FIG. 2 is a block diagram illustrating a configuration example of a communication module for optical modulation using the quantum well optical modulator in FIG. 1. The optical communication modulator module 105 of this example is
1, an electroabsorption type quantum well optical modulator 1 shown in FIG.
02 and optical fibers 103a and 103b and lenses 104a and 104b on the optical axis of the quantum well optical modulator 102.
Is fixed.

The light from the optical fiber 103a is
The light is condensed on the light absorption layer of the quantum well optical modulator 102 by the optical modulator 04a, and the light is modulated in the quantum well optical modulator 102. The light modulated and output by the quantum well optical modulator 102 is sent out to the core of the optical fiber 103b by the lens 104b and transmitted by the optical fiber 103b. Next, a communication system using the optical communication modulator module 105 using the quantum well optical modulator 102 will be described.

FIG. 6 is a block diagram showing an example of the configuration of a trunk optical communication system using the optical communication modulator module shown in FIG. The transmitting device 106 of the present example is a semiconductor laser 1 for inputting light to the modulator module 105.
07, and a drive control unit 108 for driving the modulator module 105. Light from the semiconductor laser 107 is converted into an optical signal by the modulator module 105, passes through the optical fiber 109, and is received by the light receiving unit 11 in the receiver 111.
0 is detected.

As described above with reference to FIGS. 1 to 6, in the quantum well optical modulator of this embodiment, a material having a high dielectric constant (InGaAs) is used for the well layer, and a dielectric constant is used for the barrier layer. By using a small material (ZnCdSSe) and making the ratio between the dielectric constant of the well layer and the dielectric constant of the barrier layer larger than 1.5, the binding energy of excitons in the quantum well can be increased. As a result, when the exciton binding energy is kept constant, the quantum well width can be increased as compared with the conventional case, and operation at a low voltage becomes possible.

In addition, since the optical modulator can be operated at a low voltage in this way, the optical modulator is small and can be integrated and is suitable for ultra-high-speed communication. By using this quantum well optical modulator, it is inexpensive and reliable. It is possible to create a long-distance optical communication module with high reliability and to construct an optical communication system.

The present invention is not limited to the example described with reference to FIGS. 1 to 6, and can be variously modified without departing from the gist thereof. For example, in this example, ZnCdSeTe was used for the barrier layer, but ZnCdSeTe
Also, ZnMgSSe is also a system lattice-matched to InP and has a small dielectric constant, so that ZnCdSeTe or ZnMgSSe
It is needless to say that the same effect can be obtained by using as a barrier layer. In addition, ZnCdSSe and ZnM
gSSe can also be used when GaAs is used for the substrate.

Further, GaInNA having a large dielectric constant is formed in the well layer.
When s is used, the effect of enhancing the oscillator strength of the excitons is further enhanced, and an optical modulator with higher sensitivity can be obtained. Also,
In this example, the electro-absorption type optical modulator has been described. However, it is needless to say that the same effect can be obtained by applying to an Mach-Zehnder type modulator.

[0028]

According to the present invention, in an electro-absorption type and a refractive index control type optical modulator (quantum well optical modulator) using a QCSE employing a quantum well structure, the quantum efficiency is maintained without decreasing the extinction intensity. The operating voltage can be reduced by widening the well width, and the reliability and cost of the quantum well optical modulator, the optical communication module and the optical communication system using the same can be improved.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a configuration according to the present invention of a quantum well optical modulator of the present invention.

FIG. 2 is an explanatory diagram showing a configuration example of a multiple quantum light absorption layer in a conventional quantum well optical modulator.

FIG. 3 is an explanatory diagram showing the relationship between the dielectric constant of a well layer and the dielectric constant of a barrier layer relating to exciton binding energy.

FIG. 4 is an explanatory diagram showing a comparative example of the binding energy between the material used for the quantum well optical modulator in FIG. 1 and the material used in the prior art.

5 is a block diagram showing a configuration example of a communication module for optical modulation using the quantum well optical modulator shown in FIG. 1;

6 is a block diagram showing a configuration example of a trunk optical communication system using the optical communication modulator module shown in FIG. 5;

[Explanation of symbols]

1: n-InP (100) substrate, 2: n-InP clad layer,
3: InGaAsP / ZnCdSSe multiple quantum well light absorption layer,
4: p-InP cladding layer, 5: p ⁺ -InGaAs layer, 6: n
Electrode, 7: p electrode, 8: SiO ₂ film, 9: polyimide, 1
0: antireflection film, 11: InGaAsP well layer, 12: Zn
CdSSe barrier layer, 13: InGaAs / InAlAs multiple quantum well light absorption layer, 14: InGaAs well layer, 15: InAl
As barrier layer, 101: submount, 102: electroabsorption optical modulator, 103a, 103b: optical fiber, 104
a, 104b: lens, 105: modulator module, 1
06: transmitting device, 107: semiconductor laser, 108: drive control unit, 109: optical fiber, 110: light receiver, 11
1: Receiver.

Claims (9)

(57) [Claims] 1. A quantum well absorption layer comprising a well layer and a barrier layer, wherein a degree of absorption of light incident on the quantum well absorption layer is changed by changing an electric field applied to the quantum well absorption layer. In the electro-absorption quantum well optical modulator that performs light modulation, each of the well layer and the barrier layer is formed by a ratio (ε) between a dielectric constant εw of the well layer and a dielectric constant εb of the barrier layer.
A quantum well optical modulator characterized by being formed of a combination of materials whose w / εb) is larger than 1.5. 2. A quantum well waveguide layer comprising a well layer and a barrier layer. The refractive index of light incident on the quantum well waveguide layer is changed by changing an electric field applied to the quantum well waveguide layer. In the refractive index control type quantum well optical modulator that performs optical modulation by changing, the well layer and the barrier layer each have a ratio (ε) between the dielectric constant εw of the well layer and the dielectric constant εb of the barrier layer.
A quantum well optical modulator characterized by being formed of a combination of materials whose w / εb) is larger than 1.5. 3. A quantum well waveguide layer comprising a well layer and a barrier layer. The refractive index of light incident on the quantum well waveguide layer is changed by changing an electric field applied to the quantum well waveguide layer. In a Mach-Zehnder quantum well optical modulator that modulates light by utilizing a phase change caused by a change in the refractive index, each of the well layer and the barrier layer has a dielectric constant εw of the well layer. A quantum well optical modulator formed of a combination of materials whose ratio (εw / εb) to the dielectric constant εb of the barrier layer is greater than 1.5. 4. The quantum well optical modulator according to claim 1, wherein said well layer is formed of a group III-V compound, and said barrier layer is formed of a group II-VI compound. ,
A quantum well optical modulator, wherein the ratio (εw / εb) of the dielectric constant εw of the well layer to the dielectric constant εb of the barrier layer is greater than 1.5. 5. The quantum well optical modulator according to claim 1, wherein said barrier layer is made of ZnSe, C
dSe, CdTe, ZnTe, CdS, ZnS, MgSe, MgS
Or a quantum well optical modulator comprising a mixed crystal thereof. 6. The quantum well optical modulator according to claim 1, wherein said well layer is formed of GaInNA.
A quantum well optical modulator comprising s. 7. The quantum well optical modulator according to claim 1, wherein said well layer is made of InGaAs.
A quantum well optical modulator comprising P. 8. A quantum well optical modulator according to claim 1, further comprising: means for optically coupling input light to said quantum well optical modulator; and said quantum well optical modulator. Means for optically coupling the output light to an external optical fiber. 9. A transmission means comprising the quantum well optical modulator according to claim 1 for transmitting an optical signal modulated by the quantum well optical modulator, and transmission from the transmission means. An optical communication system, comprising: waveguide means for guiding an optical signal, and receiving means for receiving an optical signal guided by the waveguide means.